Production of eta and eta' mesons via the
quasi-free proton-neutron interaction

Abstract

A comparison of the close-to-threshold total cross sections for the
eta' meson production in both the pp->pp eta'
and pn->pn eta' reactions should provide insight into
the gluonium content of eta' meson and the relevance of
quark-gluon or hadronic degrees of freedom in the creation
process.
Additionally to the mechanisms associated with meson exchanges it
is possible that the eta' meson is created from the glue
which is excited in the interaction region of the colliding
nucleons [1], and which could couple to the eta' meson
directly via its gluonic component or through its
SU(3)-flavour-singlet admixture.
The production through the colour-singlet object as suggested in
reference [1] is isospin independent and hence should lead to
the same production yield of the eta' meson in the
pn->pn eta' and pp->pp eta' reactions
after correcting for the final and initial state interaction between
the nucleons.

Investigations of the eta-meson production in nucleon
collisions allowed to conclude that close to the kinematical
threshold the creation of eta meson with total isospin
I = 0 exceeds the production with I = 1 by
about a factor of 12.
This was derived from the measured ratio of the total cross
sections for the reactions pn->pn eta and
pp->pp eta
(Reta = sigma(pn->pn eta)nbsp;/nbsp;sigma(pp->pp eta)),
which was determined to be Reta ~ 6.5
in the excess energy range between 16 MeV and
109 MeV [2].

Since the quark structure of eta and eta' mesons is
very similar we can
- by analogy to the eta meson production -
expect that in the case of dominant isovector meson exchange the
ratio Reta' should be about 6.5.
If, however, the eta' meson was produced via its
flavour-blind gluonic component from the colour-singlet glue
excited in the interaction region, the ratio Reta'
should approach unity after corrections for the interactions
between the participating baryons.

So far we have determined the close-to-threshold excitation
function for the pp->pp eta' reaction [3,4]
whereas the total cross section for the eta' meson
production in the proton-neutron interaction remains unknown.
In this report first results of the measurements of the
quasi-free pn->pn eta' reaction performed using
the newly extended COSY-11 facility [5] will be presented.